Portable electronic devices such as MP3 players, cameras, and cell phones are typically fitted with a USB port, which allows them to transfer data to or from other controllers or hosts. Using these USB ports, portable devices may conveniently draw current from personal computers to charge their batteries. In light of this convenience, engineers have developed standalone or dedicated USB chargers that expose a USB standard receptacle to portable devices, allowing them to use the same USB port to charge either from a computer or a dedicated USB charger.
USB chargers generally include a port power switch for limiting the current drawn by a connected portable device. The port power switch defines the maximum current limit for the charger. Different portable device and charging protocols expect different behavior from the port power switch when the current drawn by the portable devices exceeds the maximum current limit value. These behaviors include “Tripping” (reducing charging current to zero or near zero value), “Fold-back” (decreasing both voltage output and current output to a desired value), “Constant-Current” (decreasing voltage while maintaining charging current), or turning the switch on and off. The port power switch behaves in one of these ways to reduce the internal power dissipated by the charger.
Various cell phone and cellular chipset manufactures are not aware of an appropriate voltage-to-current slope when limiting the current to a constant value. When switching to constant current behavior, the port power switch conventionally exhibits a fixed slope for the rate of modification of current and voltage. It should be understood that the maximum current limit value varies for various charging protocols. Thus, the current systems exhibit a fixed slope, such as 10V/A or 100V/A, irrespective of the value of the maximum current limit value, such as 0.5 A or 1A.
Moreover, with growing complexity in portable devices, the battery chargers must be compatible with these portable devices and their associated multiple charging protocols. Each charging protocol defines a different maximum current limit value for the port power switch and thus, the operation of the port power switch varies for different values of current drawn. These varying characteristics of various charging protocols make current chargers incompatible with all charging protocols.
Therefore, there exists a need for a suitable method and system for current limiting the current drawn by a portable device using a power switch. The port power switch being compatible with any charging protocol.